A. Vityuk, BASF, Iselin, New Jersey
Compressed air is an important utility in refining and chemical industries and is widely used to drive pneumatic equipment and machinery. Air is also utilized as an oxygen/nitrogen source in manufacturing processes.
The drying of compressed air upstream of process equipment is crucial to avoid moisture condensation in downstream piping and to satisfy moisture specifications if air is used as the process gas. Among the available technologies, desiccant drying has been recognized as one of the most cost-efficient and versatile options, allowing water dewpoints down to –70°C (–94°F) degrees.
The desiccants that are conventionally offered into this service include activated alumina, molecular sieves and silica gels. Although these materials have been well established in industry over the years, misconceptions often remain about some of the benefits they offer in compressed air plants. This is especially pronounced for silica gels, as there are a variety of grades offered in a market with substantial variability in properties and performance characteristics in compressed air applications. This article briefly reviews the relevant benefits and challenges associated with using the most common desiccants and highlights the advantages of proprietary premium adsorbentsa,b for high energy efficiency air dehydration, comparing them to the most common silica gels desiccants offered on the market.
ADSORBENTS
Activated alumina. This is one of the most established adsorbents used in the compressed air industry. It offers a large surface area and a high-porosity matrix with good affinity toward polar compounds, especially water. The highly hydrophilic nature of the alumina surface makes it an ideal choice for adsorbing moisture. Dewpoints down to –50°C (–58°F) are accessible with activated alumina, and regeneration temperatures are normally within the 200°C–250°C (392°F–482°F) range. Major advantages of activated alumina include fairly high “in service” robustness and high stability towards alkaline components (e.g., ammonia, amines or alkali hydroxides). Combined with wide availability and a comparable low cost, activated alumina are an ideal adsorbent for heatless dryers and a good conventional choice for heat-regenerated dryers.
Molecular sieves. These sieves are crystalline aluminosilicates with frameworks stabilized by monovalent or multivalent cations from the alkali or alkaline earth group, as well as water in its as-synthesized form. The crystal water is removed by thermal treatment without damaging the crystalline structure to create the conditions for reversible water adsorption/desorption. The microporous and highly ordered structure of molecular sieves ensures very high surface areas in such materials, often in a 700 m2/g–900 m2/g range. Molecular sieves are normally used for applications where very low dewpoints down to –100°C (–148°F) are required or the compressed air must be dried at higher temperatures. The steep shape of the isotherm allows it to reach low moisture content in the product gas. However, the downside of such high affinity to moisture is the need to heat up molecular sieve beds to high temperatures to drive the water off. Regeneration temperatures in the 240°C–290°C (464°F–554°F) range are often used for efficient reactivation.
Silica gel. This amorphous and highly porous form of silicon dioxide (SiO2) exhibits high surface areas and favorable water adsorption properties. Silica gel is commercially available as granular and spherical bead material of various size ranges and has been widely used in the compressed air industry for more than 50 yr. Despite its long commercial history, users are often challenged when selecting the proper silica gel desiccant for dehydration of compressed air. This is mostly due to the abundance of various silica gel grades and types on the market as well as a poor understanding of the differences in properties and performance between these materials—this often results in an unfavorable decision towards a lower cost but less-efficient product.
Proprietary adsorbentsa,b. Unlike standard market silica gels, the author’s company’s adsorbentsa,b are alumino-silicate gels produced using a unique proprietary manufacturing process. The patented line of highly efficient adsorbents are optimized for the most energy-efficient drying.
Some of the key factors that are normally considered when selecting an adsorbent include equilibrium and dynamic water capacities, regeneration temperature (which is related to the heat of moisture adsorption on the desiccant), and durability and hydrothermal stability of the adsorbent in service. The company’s adsorbentsa,b exceed these performance parameters. High dynamic water uptake combined with comparatively low moisture desorption temperatures and outstanding hydrothermal resistance are the major benefits making these products some of the most energy-efficient in industry.
Desiccant selection for the compressed air plant is of crucial importance. Not only is it important that the proper adsorbent warrants steady and reliable operation of the dryer, but it saves the user in costs. As adsorbent replacement terms are often advised by the dryer manufacturer, the change-outs are conducted in accordance with these guidelines. The desiccant selection and service periods are normally not challenged by users and the associated costs are assumed. Part of this is due to adsorbent being viewed as a commodity and the poor differentiation between products offered on the market—this is especially relevant to silica gels, which are available in various grades, shapes, compositions and colors. A fundamental misconception is to equate the author’s company’s adsorbents to conventional silica gels or alumina-promoted silica gels.
The data provided below compares the properties and performance of the proprietary adsorbentsa,b to some of the most common silica gels offered on the market and underlines the importance and necessity of comparing adsorbents to select the proper product.
The company’s adsorbents’ performance is compared with conventional and alumina promoted silica gels that are widely offered. To make this comparison meaningful, the desiccants were divided in two types: regular grade, which are compared with the first proprietary adsorbenta (TABLE 1); and water stable grades, which are compared with the second adsorbentb (TABLE 2).
Regular grade silica gels are normally characterized by high surface area and pore volume. Products A and B in TABLE 1 exhibit high surface area pore volumes that, at first glance, point to similarities with the company’s adsorbenta. However, closer inspection shows that the high values of these commonly recognized parameters are insufficient to derive performance features. Product A is a conventional pure SiO2-based adsorbent and was shown to exhibit inferior mechanical properties compared to the adsorbenta (i.e., on average, a higher attrition rate and more pronounced aging under thermal swing cycling). It is important to highlight that lower mechanical strength and enhanced fines generation are among the common features exhibited by pure silica gel desiccants. Another meaningful disadvantage of SiO2 adsorbents is comparatively low hydrothermal stability. As desiccant is subject to multiple thermal regeneration cycles in commercial service, hydrothermal aging plays a role. While equilibrium moisture uptake on “fresh” Product A was measured as reasonably high, a pronounced decline in moisture pickup was shown when the sample was exposed to prolonged thermal regeneration cycles. The precise effect is highly dependent on the specific operating conditions of the machine.
Alumina is often added to the formulation to enhance the hydrothermal stability of silica gel adsorbents and modify some of the adsorbent properties. Product B contains about 20 wt% aluminum oxide. While silica gel desiccants promoted with a certain amount of alumina could feature enhanced stability towards liquid water, the exact response is highly dependent on the manufacturing process and the amount of alumina added in a composition. An important aspect to consider when selecting SiO2-Al2O3 desiccants with high Al2O3 content is the altered shape of the water adsorption isotherm, as compared to pure silica gels. As alumina content increases, water adsorption isotherm deviates from the isotherm typically observed for pure SiO2 and shows features more typical for alumina.
Without elaborating on the fundamental scientific nature of this phenomena, the direct consequence (relevant to operations of an industrial dryer) is reduced water dynamic capacity of the desiccant at humidities and pressures normally used for commercial compressed air dryers. While the heat of adsorption—and, therefore, moisture uptake properties—are normally not substantially affected at alumina levels of 1 wt%–5 wt%, higher content often results in lower moisture pick-up compared to pure SiO2 adsorbents. Product B with an Al2O3 content of 20 wt% reports an equilibrium uptake much lower than normally observed for the proprietary adsorbentsa,b. As a result, more adsorbent would be required (i.e., a larger vessel size) to achieve similar performance.
Another consideration when comparing SiO2 adsorbents promoted with alumina is water desorption temperature. Desiccants promoted with high amounts of alumina often exhibit higher heat of moisture adsorption and require somewhat higher regeneration temperatures. Since an increase in moisture heat of adsorption is almost directly proportional to alumina content, higher alumina content in silica gels often translates into an increase in nominal regeneration temperature. Temperatures of ~140°C are sufficient to regenerate the adsorbenta while Product B requires ~160°C for reactivation. Although some manufacturers do claim regeneration temperatures as low as 160°C degrees for high alumina-containing SiO2-based desiccants, users and dryer original equipment manufacturers (OEMs) must always execute caution when using or designing dryers with these adsorbents.
The properties of some of the available water stable desiccants are shown in TABLE 2 compared to the proprietary adsorbentb. This type of adsorbent is resistant to liquid water and often used as protective layers on top of regular grade desiccants. Product D is 99.8 wt% SiO2 pure silica gel with a surface area of 240 m2/g. The comparably low surface area coincides with the reduced equilibrium moisture uptake, which is almost 30% lower than what the adsorbentb offers. As a result, Product D would have a limited contribution to the overall desiccant bed moisture removal performance; therefore, a larger vessel size would be needed for newly designed dryers that use this adsorbent. Another related disadvantage is lower achievable dewpoint temperatures for a fixed volume of adsorbent if compared to the author’s company’s adsorbentb, even for configurations where water stable grade is used as the layer protecting a regular grade adsorbent.
Product E is alumina promoted silica gel with a reported 16 wt% Al2O3 content with a surface area of about 450 m2/g. It features a somewhat higher equilibrium moisture uptake as compared to Product D, but is inferior to the adsorbentb, which offers about 20% higher loading. An important aspect to consider when selecting a high alumina promoted silica gels-based desiccant as a protective layer is long-term hydrothermal stability. Activated alumina is known to undergo partial rehydration in the presence of liquid moisture at elevated temperatures upon extensive thermal swing cycling, conditions that are typical for the front section of the adsorbent in heat-reactivated dryers. The adsorbentb is designed to deliver outstanding performance at most severe hydrothermal conditions due to its proprietary manufacturing process, with a compositional alumina of ~3 wt%.
Takeways. Selecting the most-effective adsorbent for a compressed air dryer is important and could be very challenging if a user or OEM are not well educated on properties of the common desiccant grades offered on the market. This difficulty is especially pronounced for silica gels with an abundant product selection across applications in dryer plants. The adsorbentsa,b discussed here are high-performance desiccants optimized for air drying with very clear value propositions and competitive offerings. HP
NOTES
a Sorbead® Air R
b Sorbead® Air WS
ARTEM VITYUK is the Global Market Manager in BASF’s Adsorbents Solutions group, where he is responsible for compressed air and intermediates portfolios and leads efforts to develop and commercialize new products for sustainable processes. Dr. Vityuk earned a PhD in chemical engineering (heterogenous catalysis) from The University of South Carolina, U.S. The author can be reached at artem.vityuk@basf.com.